Why do brood combs turn black?
It doesn’t take long to discover that brood combs can turn dark as night after just one season while honey combs stay light for many years. What causes this difference? Several reasons are usually given for this phenomenon, but in truth, it is probably a combination of all them that causes brood comb to darken so quickly.
The cocoons that remain in the cell after the bees hatch are the major problem. The cocoons are extremely sticky and, try as they might, the bees cannot strip it all from the comb. Some say the darkness is caused by the feces that remains in the bottom of the cocoons. Although this may be partially true, based on what I’ve read, the bees manage to remove most—if not all—of the feces as they prepare the cell for the next generation.
What is more likely is that the sticky cocoons attract all sorts of hive debris, from dirt tracked in on bee feet (many bees times six), pollen grains, and atmospheric dust. In addition, the bees polish the insides of these cells with propolis to make the surface smooth and to take advantage of the many antimicrobial agents found in the propolis.
The color found in the dirt and debris, combined with the layers of propolis—which is usually dark—probably accounts for most of the color change.
But, you say, the inside of honey cells are brushed with propolis too. That is true, but the honey cells do not contain cocoons and they are emptied and polished seldom—usually only once a year. Brood cells, on the other hand, may be polished and reused every 21 or 22 days during the spring and summer—a huge difference.
Another difference between honey comb and brood comb is the amount of bee activity. Once a honey cell is filled the bees move on to another. But once an egg is laid in a brood cell, the uncapped larvae is fed a thousand times a day—quite a different traffic pattern.
The buildup of cocoons and propolis in brood cells is significant. Some researchers have analyzed brood comb and found that the cells become measurably smaller as the walls become thicker. If you render your own beeswax, you know how much more debris is filtered from melted brood comb than melted honey comb. Clumps of this debris, appropriately called “slumgum,” clog strainers and mesh bags, and tiny bits of it darken the liquid wax.
The question always arises whether dark comb is harmful to bees. In truth, bees love dark comb and it is often used in bait hives to attract wild swarms. I’ve heard rumors about beekeepers using black comb for twenty-five years with no ill effects.
Recently, however, there is concern about pesticide build-up in old combs, as well as the accumulation of some pathogens. Many sources now recommend rotating old black comb out of the hive every four or five years, not because of its color but to protect the hive from these pesticides and pathogens.